1. Field of the Invention
The inventions disclosed and taught herein relate generally to roller cone drill bits for use in drilling subterranean earth formations, and more specifically relate to roller cone drill bits having optimized spear point designs and blade orientations for reduced tracking and/or increased drilling performance during bit use.
2. Description of the Related Art
Roller cone rock bits are commonly used in the oil and gas industry for drilling wells. A roller cone drill bit typically includes a bit body with a threaded connection at one end for connecting to a drill string and a plurality of roller cones, typically three, attached at the opposite end and able to rotate with respect to the bit body. Disposed on each of the cones are a number of cutting elements, typically arranged in rows about the surface of the individual cones. The cutting elements may typically comprise tungsten carbide inserts, polycrystalline diamond compacts, milled steel teeth, or combinations thereof.
Significant expense is involved in the design and manufacture of drill bits to produce drill bits with increased drilling efficiency and longevity. Roller cone bits can be considered to be more complex in design than fixed cutter bits, in that the cutting surfaces of the bit are disposed on roller cones. Each of the roller cones rotates independently relative to the rotation of the bit body about an axis oblique to the axis of the bit body. Because the roller cones rotate independent of each other, the rotational speed of each cone is typically different. For a given cone, the cone rotation speed generally can be determined from the rotational speed of the bit and the effective radius of the “drive row” of the cone. The effective radius of a cone is generally related to the radial extent of the cutting elements on the cone that extend axially the farthest, with respect to the bit axis, toward the bottomhole. These cutting elements typically carry higher loads and may be considered as generally located on a so-called “drive row”. The cutting elements located on the cone to drill the full diameter of the bit are referred to as the “gage row”.
Adding to the complexity of roller cone bit designs, cutting elements disposed on the cones of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing forces. Additionally, most modern roller cone bit designs have cutting elements arranged on each cone so that cutting elements on adjacent cones intermesh between the adjacent cones. The intermeshing cutting elements on roller cone drill bits is typically desired to minimize bit balling between adjacent concentric rows of cutting elements on a cone and/or to permit higher insert protrusion to achieve competitive rates of penetration (“ROP”) while preserving the longevity of the bit. However, intermeshing cutting elements on roller cone bits substantially constrains cutting element layout on the bit, thereby, further complicating the designing of roller cone drill bits.
One prominent and recurring problem with many current roller cone drill bit designs is that the resulting cone arrangements, whether arrived at arbitrarily or using simulated design parameters, may provide less than optimal drilling performance due to problems which may not be readily detected, such as “tracking” and “slipping.” Tracking occurs when cutting elements on a drill bit fall into previous impressions formed by other cutting elements at preceding moments in time during revolution of the drill bit. This overlapping will put lateral pressure on the teeth, tending to cause the cone to align with the previous impressions. Tracking can also happen when teeth of one cone's heel row fall into the impressions made by the teeth of another cone's heel row. Slipping is related to tracking and occurs when cutting elements strike a portion of the previously made impressions and then slide into these previous impressions rather than cutting into the uncut formation.
Cutting elements on the cones do not cut effectively when they fall or slide into previous impressions made by other cutting elements. In particular, tracking is inefficient because no fresh rock is cut. It is additionally undesirable because tracking results in slow rates of penetration (ROP), detrimental wear of the cutting structures, and premature failure of the bits themselves. Slipping is also undesirable because it can result in uneven wear on the cutting elements themselves, which in turn can result in premature cutting element failure. Thus, tracking and slipping during drilling can lead to low penetration rates and in many cases uneven wear on the cutting elements and cone shell. By making proper adjustments to the arrangement of cutting elements on a bit, problems such as tracking and slipping can be significantly reduced. This is especially true for cutting elements on a drive row of a cone because the drive row generally governs the rotation speed of the cone.
Previous approaches exist for varying the orientation of cutting elements on a bit to address these tracking concerns and problems. For example, U.S. Pat. No. 6,401,839, issued to Chen, discloses varying the orientation of the crests of chisel-type cutting elements within a row, or between overlapping rows of different cones, to reduce tracking problems and improve drilling performance. U.S. Pat. Nos. 6,527,068 and 6,827,161, both issued to Singh, disclose specific methods for designing bits by simulating drilling with a bit to determine its drilling performance and then adjusting the orientation of at least one non-axisymmetric cutting element on the bit and repeating the simulating and determining until a performance parameter is determined to be at an optimum value. U.S. Pat. No. 6,942,045, issued to Dennis, discloses a method of using cutting elements with different geometries on a row of a bit to cut the same track of formation and help reduce tracking problems. However, in many drilling applications, such as the drilling of harder formations, the use of asymmetric cutting elements such as chisel-type cutting elements are not desired due to their poorer performance in these geological applications.
Prior approaches also exist for using different pitch patterns on a given row to address the tracking concerns. For example, U.S. Pat. No. 7,234,549 and U.S. Pat. No. 7,292,967 describe methods for evaluating a cutting arrangement for a drill bit that specifically includes selecting a cutting element arrangement for the drill bit and calculating a score for the cutting arrangement. This method may then be used to evaluate the cutting efficiency of various drill bit designs. In one example, this method is used to calculate a score for an arrangement based on a comparison of an expected bottom hole pattern for the arrangement with a preferred bottom hole pattern. The use of this method has reportedly lead to roller cone drill bit designs that exhibit reduced tracking over previous drill bits.
While the above approaches are considered useful in particular specific applications, typically directed to address drilling problems in a particular geologic formation, in other applications the use of such varied cutting elements is undesirable, and the use of different pitch patterns can be difficult to implement, resulting in a more complex approach to drill bit design and manufacture than necessary for addressing tracking concerns. What is desired is a simplified design approach that results in reduced tracking for particular applications without sacrificing bit life or requiring increased time or cost associated with design and manufacturing.
One method commonly used to discourage bit tracking is known as a staggered tooth design. In this design the teeth are located at unequal intervals along the circumference of the cone. This is intended to interrupt the recurrent pattern of impressions on the bottom of the hole. However, staggered tooth designs do not prevent tracking of the outermost rows of teeth, where the teeth are encountering impressions in the formation left by teeth on other cones. Staggered tooth designs also have the short-coming that they can cause fluctuations in cone rotational speed and increased bit vibration. For example, U.S. Pat. No. 5,197,555 to Estes discloses rotary cone cutters for rock drill bits using milled-tooth cones and having circumferential rows of wear resistant inserts. As specifically recited therein, “inserts on the two outermost rows are oriented at an angle in relationship to the axis of the cone to either the leading side or trailing side of the cone. Such orientation will achieve either increased resistance to insert breakage and/or increased rate of penetration.”
The inventions disclosed and taught herein are directed to new roller cone drill bit and cone designs, as well as methods and systems and drilling methods using these designs, in which variation in the roller cone spearpoint blade angle is used to reduce bit tracking.